Microelectromechanical systems (MEMS) are devices fabricated using integrated-circuit (IC) and silicon micromachining technology, with overall dimensions on the order of a millimeter and with minimum feature sizes on the order of microns. As successive layers of micromachinable materials are deposited and patterned to form one or more levels of a MEMS device, the surface topography becomes increasingly irregular and pronounced due to deposited material draping into valleys created by each underlying patterned layer of the MEMS device. This surface topography can be detrimental since it imposes limitations on photolithography, deposition, patterning and etching of subsequent layers. The resultant surface topography can further lead to mechanical interferences between moveable parts (e.g. gears, rotors, linkages, levers) on two or more functional levels of the MEMS device. Finally, the irregular surface topography can lead to the formation of stringers which are material remnants left over after an anisotropic etching step that fails to remove completely a vertically oriented portion of material in a particular layer (e.g. polysilicon) of the MEMS device. Such stringers can break loose, causing mechanical interference with moveable elements of the MEMS device, or causing an electrical short circuit in the MEMS device.
What is needed is a method for substantially planarizing the various levels of a MEMS device during formation thereof to minimize any unwanted surface topography at each level of the device. Such planarization is further complicated in the case of a MEMS device having recessed layers formed within a substrate cavity since the chemical-mechanical polishing (CMP) as used heretofore has not addressed the local planarization of recessed layers within a cavity.
Chemical-mechanical polishing has been used heretofore to provide a rapid and effective method for globally planarizing semiconductor substrates primarily for forming integrated circuits (ICs). For trench isolation of ICs, a deposited dielectric material such as silicon dioxide can be deposited to over-fill trenches separating transistors or other elements of the ICs. Chemical-mechanical polishing can then be used to polish back the silicon dioxide to level the trench with the substrate surface. Such trench isolation by CMP allows a device packing density in ICs to be increased, and also helps to prevent photolithography problems which would otherwise occur if the surface topography were to exceed the depth of focus of a stepper exposure system as successive layers of an IC structure are deposited and patterned. Thus, the goal of CMP heretofore has been to remove any excess deposited trench-fill material protruding above one or more trenches thereby planarizing the trenches so that the trench-fill material is flush with the surface of the semiconductor substrate. Any removal of material from inside the trench (termed dishing) below the substrate surface has heretofore been undesirable since it would defeat the purpose of global planarization of the substrate. In other words, it has been antithetical to planarization as known and taught heretofore to have material removed from within a trench or cavity during a chemical-mechanical polishing process step to locally planarize one or more recessed layers.
An advantage of the present invention is that a chemical-mechanical polishing step can be used to locally planarize one or more levels of a microelectromechanical (MEMS) device during formation thereof.
Another advantage of the present invention is that a multilayer MEMS device can be formed in a cavity etched into a semiconductor substrate with one or more recessed layers (e.g. sacrificial layers or polysilicon layers) of the MEMS device being locally planarized by CMP, and with the MEMS device further being encapsulated by a sacrificial material so that the semiconductor substrate can be globally planarized by CMP.
A further advantage of the present invention is that the use of CMP for locally planarizing one or more recessed layers of a MEMS device can eliminate mechanical interference between functional (i.e. moveable) elements within the MEMS device, and can further eliminate stringers that can lead to premature device failure.
These and other advantages of the method of the present invention will become evident to those skilled in the art.